1. Field of the Invention
The present invention relates to an ablation apparatus for ablating an object by laser beam (typically an excimer laser) having a non-uniform beam intensity of Gaussian distribution in one direction and a uniform beam intensity in the vertical direction. More particularly, the present invention relates to an ablation apparatus for controlling ablation of a surface of a cornea by a laser beam to correct the curvature of the cornea.
2. Description of the Related Art
Recently, some methods have been proposed for correcting the refraction of an eye by ablating the surface of the cornea to change the curvature of the cornea. In these methods, it is necessary to control the depth of the ablation area so that it is uniform. This has been accomplished by controlling the intensity distribution of the laser beam being used for ablation such that it is constant.
For example, Japanese Laid-open Patent Application No. 63-150069 (U.S. Pat. No. 4,911,711) proposes homogenizing a laser beam by using a filter having a special beam transmission distribution, and also by reflecting the laser beam.
In the filter method, the laser beam passes through a filter having a beam transmission distribution opposite to the beam intensity distribution of the laser beam, whereby the beam intensity is reduced at the part of high beam intensity area of the laser beam and a homogenized beam intensity distribution is attained.
If the laser beam from, for example, an excimer laser, has a beam section as shown in FIG. 7(a), the beam intensity distribution in the X-axis direction is uniform as shown in FIG. 7(b), and the Gaussian beam intensity distribution and like in the Y-axis direction has a maximum curvature in the central part of the laser beam as shown in FIG. 7(c). The laser beam will pass through the filter with a beam transmission distribution in the Y-axis which is low at the central part of the filter area as shown in FIG. 8(c) (in this case, the beam transmission distribution in the X-axis direction is uniform as shown in FIG. 8(b)). The intensity of the high intensity part (the central part of the intensity distribution shown in FIG. 7(c) is reduced at the low transmission part of the filter (central part of the transmission distribution shown in FIG. 8(c)).
At the low laser beam intensity part (both end parts of the intensity shown in FIG. 7(c), the laser beam passes through highly transmissive parts of the filter (both ends part of the transmission distribution shown in FIG. 8(c)), and the beam intensity is only slightly reduced. Consequently, the intensity of the transmitted laser beam becomes the same at the low part (both end portions) and the high part (central portion), and uniformity of the beam intensity is attained.
The method to homogenize a laser beam by reflecting it distributes the laser beam to a plurality of parts and synthesizes them so that the beam intensity is homogenized. Such a homogenizing apparatus is shown in FIG. 9. The apparatus comprises a central triangular optical prism 2a, two smaller outer triangular prisms 2b, 2c, first and second beam splitting interfaces 3a, 3b, an inlet assembly of spaced reflectors 4a, 4b to divide the laser beam, and outer pairs of reflectors 5a, 5b and 6a, 6b.
The reflectors 4a, 4b each reflect an outer one-third portion of the beam with an expanded height dimension H, to the direction of the reflectors 5a, 6a for further reflection by reflectors 5a, 6a to offset the outer portions from the path of the central one-third portion. At the location of the operative part of the beam splitter 3a, the reflector 5b deflects the divided upper one-third fraction into additive relation with the central one-third fraction. At the location of the operative part of the beam splitter 3b, the reflector 6b reflects the divided lower one-third fraction into an additive relation with the central one-third, and with the already added upper one-third fraction.
FIG. 10 depicts the functional result of what has been described in connection with FIG. 9. The Gaussian intensity profile P of a beam shown by dashed lines includes an upper portion that is picked off and transmitted by reflectors 4a, 5a, 5b for addition at beam splitting interface 3a with the central portion. The displaced upper portion is indicated by the alternate long and short dash line P'.
Similarly, the lower portion of the profile P" indicated by alternate long and short dash line is picked off and transmitted by reflectors 4b, 6a, 6b for addition at beam splitting interface 3b with the already combined upper and central portions. The net result is a beam output which has the H/3 dimension of the central one-third portion and which has an added intensity distribution substantially as indicated by the solid-line profile P.sub.R.
Another proposed method is shown in Japanese Laid-open Patent Application No. 63-289519 which uses a cylindrical lens array including a dense array of small, cylindrical lenses arranged parallel to the X-direction. FIG. 11 depicts an arrangement where the laser beam arrives at an irradiation surface S, by passing through a toric lens 7 and the cylindrical lens array 8.
After the laser beam is converged by the toric lens 7 having a strong beam converging nature in the Y-direction and a weak beam converging nature in the X-direction, the beam passes to the cylindrical lens array 8. The refraction of the laser beam in the Y-direction by the cylindrical lens array is randomized, and the intensity distribution is averaged equally on the irradiation surface S.
There are several problems with the methods mentioned above. In the method which uses the beam transmission filter having a special beam transmission distribution, it is difficult to produce the filter having the proper curvature and a unified intensity distribution may not be obtained because the beam transmission distribution is inaccurate. Also, if the intensity distribution of the laser beam is changed, or the beam axis is misaligned, the intensity distribution of the laser beam could not be counterbalanced by the beam transmission distribution of the filter. Also, there is a problem in that the degree of beam transmission of the high beam intensity area must be reduced to conform with the low beam intensity area, and the loss of energy is unacceptable.
In the homogenizing method using laser beam reflection by a plurality of mirrors, the structure becomes complex and it takes much time to adjust an arrangement of components. If the intensity of the laser beam is changed or the beam axis is misaligned, a uniform intensity distribution cannot be obtained and the energy loss is unacceptable when the laser beam is recombined after being divided by the beam splitter.
In the method using the cylindrical lens array, the production of the cylindrical lens array is complex and requires much time to produce.